1. Field of the Invention
This invention relates to a piezoelectric actuator for use in a strong electric field, at a high compressive stress and in a broad temperature range, and to a method for driving the same.
2. Description of the Related Art
A piezoelectric actuator using a piezoelectric body as a driving source exhibits a correct display amount. Therefore, the use of the piezoelectric actuator as a driving source for driving a valve body of a liquid injection apparatus such as an injector has been examined.
PZT (lead zirco-titanate) type oxides having an ABO3 type perovskite structure are known as piezoelectric bodies as described in Japanese Unexamined patent Publication (Kokai) No. 9-55549, for example.
Such piezoelectric bodies include a tetragonal system and a rhombohedral system depending on their compositions. It has been clarified that a morphotropic phase boundary (MPB) exists between these crystal phases. It has also been clarified that the piezoelectric characteristics such as the dielectric constant, the piezoelectric strain constant, etc, become maximal on the MPB.
The Japanese Unexamined Patent Publication (Kokai) No. 9-55549 described above discloses that temperature dependence of displacement and that of the dielectric constant can be reduced when a material that is tetragonal at a higher temperature in the proximity of the MPB between the tetragonal and rhombohedral phases and is rhombohedral when a lower temperature is selected. When such a material is selected, temperature characteristics can be improved in a weak electric field and at a low compressive stress. and at a low compressive stress.
However, it has been required in recent years for the actuators to exhibit a small displacement at a high electric field and a high compressive stress and in a broad temperature range, and the prior art technologies described above cannot alone satisfy such needs.
In view of the problems of the prior art technologies described above, this invention contemplates to provide a piezoelectric actuator exhibiting a small displacement in a broad range of use conditions of temperatures, electric fields and compressive stresses and a driving method of the same.
According to a first aspect of the invention, there is provided a piezoelectric actuator using a piezoelectric body undergoing displacement upon application of a voltage as a driving source, characterized in that a crystalline structure of the piezoelectric body substantially exists on a tetragonal system side outside of a morphotropic phase boundary (MPB) between the tetragonal system and a rhombohedral system under a condition of the lowest temperature, the smallest electric field and the maximum compressive stress that are used.
The piezoelectric actuator according to the invention uses a piezoelectric body having a specific crystalline structure under the specific condition described above. In other words, the invention stipulates the crystalline structure of the piezoelectric body under the three specific conditions, that is, the lowest temperature in the temperature range in which the piezoelectric actuator is used, the lowest electric field in the range in which the piezoelectric actuator is used and the maximum compressive stress in the range employed.
The crystalline structure of the piezoelectric body so stipulated substantially exists on the tetragonal system side outside of the morphotropic phase boundary (MPB) between the tetragonal system and the rhombohedral system as described above. When this requirement is satisfied, the crystalline structure of the piezoelectric body does not change to the rhombohedral system even when the temperature, electric field and compressive stress used change, but always retains the tetragonal system. In the piezoelectric body described above, the tetragonal system is superior to the rhombohedral system in reliability.
Therefore, even when the use conditions of the piezoelectric actuator such as the temperature, the electric field and the compressive stress change, the crystalline structure of the piezoelectric body does not change but remains the tetragonal system, and can maintain relatively stable characteristics.
The invention sets the crystalline structure of the piezoelectric body so that it exists in the proximity of the morphotropic phase boundary (MPB) between the tetragonal system and the rhombohedral system and moreover, substantially on the tetragonal system side while the three specific conditions described above are satisfied. In this way, even when the use conditions change, the piezoelectric body can keep the tetragonal system relatively approximate to MPB and its characteristics in an excellent condition.
Therefore, the invention can provide a piezoelectric actuator having a small change of displacement even when the ranges of the use conditions of the temperature, the electric field and the compressive stress are broad.
According to a second aspect of the invention, there is provided a method for driving a piezoelectric actuator using a piezoelectric body undergoing displacement upon application of a voltage as a driving source, characterized in that the piezoelectric body is constituted so that a crystalline structure thereof substantially exists on a tetragonal system side outside of a morphotropic phase boundary (MPB) between the tetragonal system and a rhombohedral system under a condition of the lowest temperature, the lowest electric field and the maximum compressive stress that are used, and that the piezoelectric body is driven under a condition of at least the lowest temperature, at least the lowest electric field and not greater than the maximum compressive stress.
As described above, the method for driving the piezoelectric actuator according to the invention uses the piezoelectric body having the specific crystalline structure under the three specific conditions when the piezoelectric actuator is used. When the piezoelectric body is driven under the conditions of at least the lowest temperature, at least the lowest electric field and not greater than the maximum compressive stress described above, the piezoelectric body can always keep the crystalline structure of the tetragonal system.
Even when the use conditions change, the tetragonal system relatively near MPB can be kept, and the characteristics of the piezoelectric body can be kept under the excellent condition. Therefore, this driving method can minimize the change of the displacement of the piezoelectric body to be driven even when the ranges of the use conditions of the temperature, the electric field and the compressive stress are broad.